Control of Hierarchically Ordered Positive and Negative Replicas of Wood Cellular Structures by Surfactant-Directed Sol-
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Control of Hierarchically Ordered Positive and Negative Replicas of Wood Cellular Structures by Surfactant-Directed Sol-Gel Mineralization Yongsoon Shin,1* Jun Liu,2 Li-Qiong Wang,1 Jeong Ho Chang,1 William D. Samuels,1 Larry R. Pederson,1 Gregory J. Exarhos1 1 2
Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K2-44, Richland, WA 99352 Sandia National Laboratory, Albuquerque, NM 87123
ABSTRACT We here report the synthesis of ordered ceramic materials with hierarchy produced by an in-situ mineralization of ordered wood cellular structures with surfactant-templated sol-gel at different pH. At low pH, a silicic acid is coated onto inner surface of wood cellular structure and it penetrates into pores left, where degraded lignin and hemicellulose are leached out, to form a positive replica, while at high pH the precipitating silica particles due to fast condensation clog the cells and pit structures to form a negative replica of wood. The calcined monoliths produced in different pHs contain ordered wood cellular structures, multi-layered cell walls, pits, vessels well-preserved with positive or negative contrasts, respectively. The surfactant-templated mineralization produces ordered hexagonal nanopores with 20Å in the cell walls after calcination. INTRODUCTION Self-assembly approaches to the synthesis of ordered inorganic oxide architectures are of great interest since such materials show marked improvement in separation efficiency, [1] catalytic behavior, [2,3] and tailored fluid transport making them attractive for bioengineering applications [4]. A cadre of biological and non-biological based approaches have been used to both self-assemble nanoparticle building blocks and then assemble these fundamental units into larger structures [5]. However, post-assembly removal of the templating agents used to form resulting inorganic architectures restricts the size of the ordered pore channels to a few hundred angstroms [6]. Synthetic materials with ordered structures on micro, meso, and macroporous dimensions have not been successfully prepared even though considerable efforts have been expended that invoke that either temperature-programmed hydrothermal processing [7] or dual templating approaches [8]. Ordered porous materials with larger pore sizes are of considerable interest since they could provide for more rapid mass transport to the active sites thereby improving reaction kinetics. Previous efforts to silicify, [9,10] or to carbonize [11] wood tissues produced small segments of the negative replica of the wood structure in a porous ceramic form, but did not reproduce the structural nuances intrinsic of the natural wood material. Rather than simple deposition of a ceramic phase within the wood interstices or infiltration of the ceramic into the unoccupied void space as seen in the earlier publications, work reported here demonstrates chemical control of the deposition process through solution pH and the presence of a templating agent. This approach allows formation of either positive or negative replicas o
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